Logistics. It isn't practical to have someone shuffle a deck if they aren't allowed to see any of the cards they're shuffling, so if you're using playing cards to assign roles, at least the dealer will know whether or not the players are all red.

One possible solution would be to have a PDA or smartphone assign the roles, and pass it around. If you do it this way, you could also have a small chance that one player is given the exact real answer. (But red players could falsely claim that they have it.)